Prius inverter question

Hi everyone, I am a new member and this is my first post here.
I have a question I hope someone can answer. What is the purpose/function of the three large capacitors inside the THS-II inverter/converter?

hobbit can you enlarge for the Biologists, out there! Chopper and DC ripple that has vague convergence from a few courses a long time ago. I would like to understand what you said (again?)
[/b]

Click to expand...

All large inverter systems have big capacitors on the DC input...

I work on DC power systems that power large telephone systems.. Many of those inverters have a capacitor charge start button built in to charge up the capacitors before starting the inverter. The Prius has a third relay in the battery pack that charges the capacitors through a resistor. Otherwise there would be a huge current surge on startup.

Also the way the inverter steps up the voltage from 200 to 500volts is essentially a switching voltage doubler.. A somewhat similar concept was used in high voltage tube type power supplies many years ago.. and uses the capacitor bank for that purpose..

The large capacitors serve as a local energy reservoir and smooths out the current draw by the inverter.. Otherwise there would be heavy current spikes as the inverter "transistors" (triacs) (or choppers as referred to above) trigger to make 3 phase AC.

Below is a pic of my home UPS system which is a scaled down version of one type of system supporting telephone switches. One system we service has 18,000 lbs of batteries supporting the telephone system.. It has a steady state current draw of around 395amps at 48 volts.

hobbit can you enlarge for the Biologists, out there! Chopper and DC ripple that has vague convergence from a few courses a long time ago. I would like to understand what you said (again?)
[/b]

Click to expand...

Think of a capacitor as a sort of electrical balloon or storage tank. Electrons can be squeezed into a capacitor under pressure, which is called voltage (volts) in the electrical world. When connected to a circuit, these same electrons flow out (current or amperage), just like opening a pressure tank or releasing a balloon.

There is a bank of super-capacitors in the Prius for the electrical brake system. They store charge for running the brakes in the event of an electrical supply failure, kind of like the air tank on air brakes (sort of).

A common use of smaller capacitors is for filtering electrical noise. When pulses, spikes, and other disturbances travel down a wire, a capacitor can be used to smooth out the flow. Once again the pressure tank analogy works. It's like the little expansion chambers used with plumbing, so when you slam off a faucet the flowing water has somewhere to go. Otherwise you get that water hammer sound.

The big capacitors in the inverter are probably for a voltage multiplier circuit (I haven't looked at any schematics, so I'm just guessing). The design of the Prius strikes a balance between high voltage for motor efficiency and lower voltage for battery cost. A switching voltage multiplier works like this:

1) Several capacitors are connected in parallel across the source voltage. In our example, lets say three capacitors are connected to a 200V battery. Each of these caps charges to 200V.

2) The caps are disconnected from the battery. Each is still charged to 200V. They are then connected in series, or end to end, so the combined voltage is 600V. This 600V is used to power the motor, which rapidly drains the charge. As the charge drops, so does the voltage. When the voltage gets too low, the system switches back to step 1 above, and the cycle repeats. This happens very quickly.

Note that 600V is higher than the 500V used in the Prius for the motor. 600V is the absolute top voltage you can get from three 200V caps without any load. In practice, the voltage of the caps is sliding up and down continuously as charge is supplied and used. 500V becomes the working voltage, or the voltage the inverter is able to continuously supply.

Think of a capacitor as a sort of electrical balloon or storage tank. Electrons can be squeezed into a capacitor under pressure, which is called voltage (volts) in the electrical world. When connected to a circuit, these same electrons flow out (current or amperage), just like opening a pressure tank or releasing a balloon.

There is a bank of super-capacitors in the Prius for the electrical brake system. They store charge for running the brakes in the event of an electrical supply failure, kind of like the air tank on air brakes (sort of).

A common use of smaller capacitors is for filtering electrical noise. When pulses, spikes, and other disturbances travel down a wire, a capacitor can be used to smooth out the flow. Once again the pressure tank analogy works. It's like the little expansion chambers used with plumbing, so when you slam off a faucet the flowing water has somewhere to go. Otherwise you get that water hammer sound.

The big capacitors in the inverter are probably for a voltage multiplier circuit (I haven't looked at any schematics, so I'm just guessing). The design of the Prius strikes a balance between high voltage for motor efficiency and lower voltage for battery cost. A switching voltage multiplier works like this:

1) Several capacitors are connected in parallel across the source voltage. In our example, lets say three capacitors are connected to a 200V battery. Each of these caps charges to 200V.

2) The caps are disconnected from the battery. Each is still charged to 200V. They are then connected in series, or end to end, so the combined voltage is 600V. This 600V is used to power the motor, which rapidly drains the charge. As the charge drops, so does the voltage. When the voltage gets too low, the system switches back to step 1 above, and the cycle repeats. This happens very quickly.

Note that 600V is higher than the 500V used in the Prius for the motor. 600V is the absolute top voltage you can get from three 200V caps without any load. In practice, the voltage of the caps is sliding up and down continuously as charge is supplied and used. 500V becomes the working voltage, or the voltage the inverter is able to continuously supply.

Hope this makes it more clear,
Tom
I like how the inverter has a view out the window.

The big capacitors in the inverter are probably for a voltage multiplier circuit (I haven't looked at any schematics, so I'm just guessing). The design of the Prius strikes a balance between high voltage for motor efficiency and lower voltage for battery cost. A switching voltage multiplier works like this:

1) Several capacitors are connected in parallel across the source voltage. In our example, lets say three capacitors are connected to a 200V battery. Each of these caps charges to 200V.

2) The caps are disconnected from the battery. Each is still charged to 200V. They are then connected in series, or end to end, so the combined voltage is 600V. This 600V is used to power the motor, which rapidly drains the charge. As the charge drops, so does the voltage. When the voltage gets too low, the system switches back to step 1 above, and the cycle repeats. This happens very quickly.

Note that 600V is higher than the 500V used in the Prius for the motor. 600V is the absolute top voltage you can get from three 200V caps without any load. In practice, the voltage of the caps is sliding up and down continuously as charge is supplied and used. 500V becomes the working voltage, or the voltage the inverter is able to continuously supply.

Hope this makes it more clear,
Tom
I like how the inverter has a view out the window.

Tom
[/b]

Click to expand...

Tom,

Your reply gets to the basis of my question. Do the capacitors in the Prius inverter supply voltage to the electric motor for propulsion or are they in place strictly for suppressing voltage spikes? Your answer would seem to support the theory that they do momentarily power the motor. However an earlier reply stated the opposite.

Your reply gets to the basis of my question. Do the capacitors in the Prius inverter supply voltage to the electric motor for propulsion or are they in place strictly for suppressing voltage spikes? Your answer would seem to support the theory that they do momentarily power the motor. However an earlier reply stated the opposite.
[/b]

Click to expand...

George, I don't know the answer to your question, as I have never seen the schematic of the Prius inverter. Even the fact that there are three caps doesn't tell us, since a three phase motor could have three snubber capacitors, or it could be a switched capacitor voltage multiplier.

Maybe someone on PC can answer this, or perhaps post the part of the schematic showing the caps in question?

Hey, it looks like you've got the same kind of microferrups I do:http://techno-fandom.org/~hobbit/pix/prius-ferrups.jpg
Is it the 450va one, or a larger version?
.
I really like this type of UPS; despite the inefficiency of the fact
that it's a constant-voltage transformer, it delivers *very* clean
output.
.
_H*

Hey, it looks like you've got the same kind of microferrups I do:http://techno-fandom.org/~hobbit/pix/prius-ferrups.jpg
Is it the 450va one, or a larger version?
.
I really like this type of UPS; despite the inefficiency of the fact
that it's a constant-voltage transformer, it delivers *very* clean
output.
.
_H*
[/b]

In this case, it was 1> my "ballast" for my Tour de Sol runs since it
weighs almost as much as a passenger would, and 2> was handy for
supplying AC power for tent lighting [before we got building power
run out to the TdS area] and various silly experiments, as detailed
in http://priuschat.com/index.php?showtopic=19503
The UPS also contains another 66 amp-hours of 12V lead-acid inside
it, so not only can the car maintain the UPS, the UPS could jump the
car if needed.
.
I've got a much smaller UPS rigged up in a similar fashion that rides
around in the seekrit luggage compartment underneath, that can be
hooked up the same way when needed, but it's only modified-squarewave
output so won't run waveform-sensitive items [or at least not
very happily].
.
As far as the original inverter and capacitors question... the
balloon analogy is good, but let's clarify that the caps are *not*
used as a traditional switched-capacitor voltage doubler. They
simply filter the output from an inductive buck/boost converter
[boost when 200V batteries supply 500V rail to motors, buck when
motors produce indeterminately high regen/charging power and
something needs to regulate charge current].
.
I thought there was a diagram of the thing under some of the Toyota
THS-II web material, but the best I can find is in a little piece of
Techinfo. Refer to the diagram on page 6 [labeled page TH-7] ofhttp://techno-fandom.org/~hobbit/cars/TIS/ncf/thsiicon.pdf and take
note of the circuitry around what they call the "reactor". And, of
course, where the capacitors are shown. Simple filtering.
.
_H*

As far as the original inverter and capacitors question... the
balloon analogy is good, but let's clarify that the caps are *not*
used as a traditional switched-capacitor voltage doubler. They
simply filter the output from an inductive buck/boost converter
[boost when 200V batteries supply 500V rail to motors, buck when
motors produce indeterminately high regen/charging power and
something needs to regulate charge current].
[/b]

Click to expand...

We have to use the same setup with industrial variable frequency motor drives. A typical 6-pulse VFD can produce in the range of 80% harmonic ramping to load. New 12/18 pulse VFD's are much more respectful of PF correction.

In industry it's most common to pair LC - called a "reactor" - to line side to ensure PF correction. It's also done on load side if the motor isn't rated for "inverter duty," to avoid corona on the stator/rotor windings. You'd be surprised how often a large company specs a pricey TEFC motor but saves a nickel by not specifying Inverter Duty.

I received an answer from Toyota engineer Akira Kawahashi. The three large capacitors, called the main capacitor, is an energy storage device and supplies system voltage and current to the motor generator. The main capacitor also provides voltage spike protection. The smaller capacitor, called the filter capacitor is for filtering only. This information can also be found in SAE document 01332845.

The posts on this forum made it seem like there were two 12v batteries in the back of the prius. I just put up a duplicate post looking for an answer at to that question and the easiest way to get to the 12v battery (if there is only one).

Conversations here talked about a 12v accessory battery and a 12v traction control battery.

ANYWAY, I opened the back of my Prius and found only one 12v battery. I ran 8 guage jumper cables from that battery to my inverter and voila, I have power. I felt the cables get warm (not hot), so just to be safe I picked up a set of 6 guage cables. I will put better ends on one pair-side of the cables so they can screw onto the inverter.

I would appreciate any advice people give, do you use a 6 or 8 guage wire on yours?

About PriusChat

Founded in 2003, PriusChat has been the go-to spot for Prius, hybrid, and EV discussion for over 10 years. PriusChat is the one of the largest privately-owned car forums left on the internet. We hope you'll join our home to educated & critical discussion, falling outside of the fray of Internet trolls & unconstructive discussion.